Systems and methods for examining the electrical characteristic of cardiac tissue

ABSTRACT

Systems and methods examine heart tissue morphology using three or more spaced apart electrodes, at least two of which are located within the heart in contact with endocardial tissue. The systems and methods transmit electrical current through a region of heart tissue lying between selected pairs of the electrodes, at least one of the electrodes in each pair being located within the heart. The systems and methods derive the electrical characteristic of tissue lying between the electrode pairs based, at least in part, upon sensing tissue impedances. The systems and methods make possible the use of multiple endocardial electrodes for taking multiple measurements of the electrical characteristics of heart tissue. Multiplexing can be used to facilitate data processing. The systems and methods also make possible the identification of regions of low relative electrical characteristics, indicative of infarcted tissue, without invasive surgical techniques.

FIELD OF THE INVENTION

[0001] The invention relates to systems and methods for mapping theinterior regions of the heart for treatment of cardiac conditions.

BACKGROUND OF THE INVENTION

[0002] Physicians examine the propagation of electrical impulses inheart tissue to locate aberrant conductive pathways. The aberrantconductive pathways constitute peculiar and life threatening patterns,called dysrhythmias. The techniques used to analyze these pathways,commonly called “mapping,” identify regions in the heart tissue, calledfoci, which are ablated to treat the dysrhythmia.

[0003] Conventional cardiac tissue mapping techniques use multipleelectrodes positioned in contact with epicardial heart tissue to obtainmultiple electrograms. Digital signal processing algorithms convert theelectrogram morphologies into isochronal displays, which depict thepropagation of electrical impulses in heart tissue over time. Theseconventional mapping techniques require invasive open heart surgicaltechniques to position the electrodes on the epicardial surface of theheart.

[0004] Furthermore, conventional epicardial electrogram processingtechniques used for detecting local electrical events in heart tissueare often unable to interpret electrograms with multiple morphologies.Such electrograms are encountered, for example, when mapping a heartundergoing ventricular tachycardia (VT). For this and other reasons,consistently high correct foci identification rates (CIR) cannot beachieved with current multi-electrode mapping technologies.

[0005] Researchers have taken epicardial measurements of the electricalresistivity of heart tissue. Their research indicates that theelectrical resistivity of infarcted heart tissue is about one-half thatof healthy heart tissue. Their research also indicates that ischemictissue occupying the border zone between infarcted tissue and healthytissue has an electrical resistivity that is about two-thirds that ofhealthy heart tissue. See, e.g., Fallert et al., “Myocardial ElectricalImpedance Mapping of Ischemic Sheep Hearts and Healing Aneurysms,”Circulation, Vol. 87, No. 1, January 1993, 199-207.

[0006] This observed physiological phenomenon, when coupled witheffective, non-intrusive measurement techniques, can lead to cardiacmapping systems and procedures with a CIR better than conventionalmapping technologies.

SUMMARY OF THE INVENTION

[0007] A principal objective of the invention is to provide improvedprobes and methodologies to examine heart tissue morphology quickly,accurately, and in a relatively non-invasive manner.

[0008] One aspect of the invention provides systems and methods forexamining heart tissue morphology using three or more spaced apartelectrodes, at least two of which are located within the heart incontact with endocardial tissue. The systems and methods transmitelectrical current through a region of heart tissue lying betweenselected pairs of the electrodes, at least one of the electrodes in eachpair being located within the heart. Based upon these currenttransmissions, the systems and methods derive the electricalcharacteristic of tissue lying between the electrode pairs.

[0009] This electrical characteristic (called the “E-Characteristic”)can be directly correlated to tissue morphology. A low relativeE-Characteristic indicates infarcted heart tissue, while a high relativeE-Characteristic indicates healthy heart tissue. IntermediateE-Characteristic values indicate the border of ischemic tissue betweeninfarcted and healthy tissue.

[0010] According to this aspect of the invention, the systems andmethods derive the tissue E-Characteristic of at least two differenttissue sites within the heart without altering the respective positionsof the endocardial electrodes. The systems and methods make possible thedifferentiation of regions of low relative E-Characteristic from regionsof high relative E-Characteristic, without invasive surgical techniques.

[0011] Another aspect of the invention provides systems and methods thatgenerate a display showing the derived E-Characteristic in spatialrelation to the location of the examined tissue regions. This aspect ofthe invention makes possible the mapping of the E-Characteristic ofheart tissue to aid in the identification of possible tissue ablationsites.

[0012] How the E-Characteristic is expressed depends upon how theelectrical current is transmitted by the electrode pair through theheart tissue.

[0013] When one of the electrodes in the pair comprises an indifferentelectrode located outside the heart (i.e., a unipolar arrangement), theE-Characteristic is expressed in terms of tissue impedance (in ohms).When both electrodes in the pair are located inside the heart (i.e., abipolar arrangement), the E-Characteristic is expressed in terms oftissue resistivity (in ohm·cm).

[0014] In a preferred embodiment, the systems and methods employelectrodes carried by catheters for introduction into contact withendocardial tissue through a selected vein or artery. The systems andmethods transmit electric current and process information through signalwires carried by the electrodes. The electrodes can be connected to amultiplexer/demultiplexer element, at least a portion of which iscarried by the catheter, to reduce the number of signal wires thecatheter carries, and to improve the signal-to-noise ratio of the dataacquisition system.

[0015] Other features and advantages of the inventions are set forth inthe following Description and Drawings, as well as in the appendedClaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a plan view, with portions in section, of a system forexamining and mapping heart tissue morphology according to the featuresof the invention, shown deployed for use within the heart;

[0017]FIG. 2 is a plan view, with portions in section, of the systemshown in FIG. 1 in the process of being deployed for use within theheart;

[0018]FIG. 3 is a view of the mapping probe and process controllerassociated with the system shown in FIG. 1;

[0019]FIG. 4 is an enlarged perspective view of an electrode carryingspline associated with the probe shown in FIG. 1;

[0020]FIG. 5 is a cross sectional view of an alternative embodiment ofan electrode that can be associated with the probe shown in FIG. 1,taken generally along line 5-5 in FIG. 6;

[0021]FIG. 6 is an enlarged perspective view of an alternativeembodiment of an electrode carrying spline that can be associated withthe probe shown in FIG. 1;

[0022]FIGS. 6A to 6C and associated catheter tube are views of aflexible electrode support body that can carry the electrodes anddeployed in the heart according to the invention;

[0023]FIG. 7 is a schematic view of the current generator module andswitching element of the process controller for the system shown in FIG.1;

[0024]FIG. 8 is a diagrammatic view of the current generator module andswitching element when operated in a Unipolar Mode;

[0025]FIG. 9 is a diagrammatic view of the current generator module andswitching element when operated in a Bipolar Two Electrode Mode;

[0026]FIG. 10 is a diagrammatic view of the current generator module andswitching element when operated in a Bipolar Four Electrode Mode;

[0027]FIGS. 11 and 12 are schematic views of the details of theswitching element shown in FIGS. 7 to 10;

[0028]FIG. 13 is a schematic view of the signal processor module of theprocess controller for the system shown in FIG. 1;

[0029]FIG. 14 is a schematic view of the E-Characteristic computingsystem of the signal processor module shown in FIG. 13;

[0030]FIG. 15 is an illustrative, idealized display of the absolutetissue E-Characteristic values derived by the system shown in FIG. 14arranged in spatial relation to a region of the heart;

[0031]FIG. 16 is a flow chart showing the operation of the system thatarranges the derived absolute tissue E-Characteristic values into groupsof equal values;

[0032]FIG. 17 is a representative display of the groups of equalE-Characteristic values derived by the system shown in FIG. 16 arrangedin spatial relation to a region of the heart; and

[0033]FIG. 18 is a diagrammatic view of an alternative embodiment of acontroller that can be used in association with the system shown in FIG.1;

[0034]FIG. 19 is a diagrammatic view of the pacing module that thecontroller shown in FIG. 18 includes;

[0035]FIG. 20 is a diagrammatic view of the host processing unit andelectrogram signal processing module with which the controller shown inFIG. 18 is associated;

[0036]FIG. 21A is a view of four representative electrograms that can beused to compute electrogram events;

[0037]FIG. 21B is a flow chart showing the methodology for computing anelectrogram event for processing by the controller shown in FIG. 18;

[0038]FIG. 22 is a flow chart showing the operation of the means forconstructing an iso-display of the computed electrogram event;

[0039]FIG. 23 is a representative iso-chronal display;

[0040]FIG. 24 is a flow chart showing the operation of the means forconstructing an iso-conduction display of the computed electrogramevent;

[0041]FIG. 25 is a representative iso-conduction display;

[0042]FIG. 26 is a flow chart showing the operation of the means formatching iso-E-Characteristics with iso-conduction information;

[0043]FIG. 27 is a representative display of the matchediso-E-Characteristics and iso-conduction information;

[0044]FIG. 28 is a flow chart showing the operation of the means fordetecting a possible ablation site based upon the information obtain inFIG. 26;

[0045]FIG. 29 is a representative display of the matchedIso-E-Characteristics and iso-conduction information, after selection ofa threshold value, identifying a potential ablation site; and

[0046]FIG. 30 is a plan view of an ablation probe being used inassociation with the system shown in FIG. 1.

[0047] The invention may be embodied in several forms without departingfrom its spirit or essential characteristics. The scope of the inventionis defined in the appended claims, rather than in the specificdescription preceding them. All embodiments that fall within the meaningand range of equivalency of the claims are therefore intended to beembraced by the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] FIGS. 1 to 3 show the components of a system 10 for examiningheart tissue morphology. FIG. 1 shows the system 10 deployed and readyfor use within a selected region 12 inside a human heart.

[0049] As FIGS. 1 and 2 show, deployment of the system 10 does notrequire invasive open heart surgical techniques. Instead, the system 10includes an introducer 14 and an outer guide sheath 16 that togetherdirect a multiple electrode probe 18 into the selected region 12 withinthe heart through a selected vein or artery. FIG. 3 shows the probe 18in its entirety.

[0050] The physician uses the probe 18 in association with a processcontroller 20 (see FIG. 3) to take multiple, sequential measurements ofthe transmission of electrical current by heart tissue. From these, theE-Characteristic of the tissue is derived. In the illustrated andpreferred embodiment, these measurements are used to assist thephysician in identifying appropriate ablation sites within the heart.

[0051]FIG. 1 and the other figures generally show the system 10 deployedin the left ventricle of the heart. Of course, the system 10 can bedeployed in other regions of the heart, too. It should also be notedthat the heart shown in the Figures is not anatomically accurate. TheFigures show the heart in diagrammatic form to demonstrate the featuresof the invention.

I. Non-invasive System Deployment

[0052] As FIG. 1 shows, the introducer 14 has a skin-piercing cannula22. The cannula 22 establishes percutaneous access into the selectedvein or artery (which is typically the femoral vein or artery). Theother end of the introducer 14 includes a conventional hemostatic valve24.

[0053] The physician advances the outer guide sheath 16 through theintroducer 14 through the vein or artery into the selected heart chamber12. The hemostatic valve 24 yields to permit the introduction of theouter guide sheath 16 through it, but otherwise conforms about the outersurface of the sheath 16, thereby maintaining a fluid tight seal.

[0054] Preferably, the guide sheath 16 includes a precurved distal tipregion 26, like a conventional “pig tail” catheter. The precurved distaltip region 26 assists in steering the guide sheath 16 into positionwithin the heart chamber 12.

[0055] The physician advances the probe 18 through the handle 28 of theouter sheath 16. The handle 28 includes a second conventional hemostaticvalve 30 that yields to permit the introduction of the flexible body 32of the mapping probe 18 through it. At the same time, the valve 30conforms about the outer surface of the body 22 to maintain a fluidtight seal.

[0056] Further details of the deployment and use of the introducer 14and guide sheath 16 to establish a pathway for the probe 18 are setforth in pending U.S. patent application Ser. No. 08/033,641, filed Mar.16, 1993, entitled “Systems and Methods Using Guide Sheaths forIntroducing, Deploying, and Stabilizing Cardiac Mapping and AblationProbes.”

II. The Tissue Examination Probe

[0057] As FIGS. 1 and 3 best show, the probe 18 has a handle 34 attachedto the proximal end of the flexible catheter body 32. The distal end ofthe catheter body 32 carries a three dimensional structure 36. In FIGS.1 and 3, the structure 36 takes the form of a basket. It should beappreciated that other three dimensional structures could be used.

[0058] The three dimensional basket structure 36 carries an array ofelectrodes 38.

[0059] As FIG. 1 shows, when deployed inside the heart chamber 12, thebasket structure 36 holds the electrodes 38 in intimate contact againstthe endocardial surface of the heart chamber 12.

[0060] The catheter body 32 passes through the outer guide sheath 16.The sheath 16 has an inner diameter that is greater than the outerdiameter of the catheter body 32. As a result, the sheath 16 can slidealong the catheter body 32. The sheath handle 28 helps the user slidethe sheath 16 along the catheter body 32.

[0061] As FIG. 2 shows, forward movement of the sheath handle 28 (i.e.,toward the introducer 14) advances the distal end of the slidable sheath16 upon the basket structure 36. In this position, the slidable sheath16 captures and collapses the basket structure 36, entirely enclosingthe basket structure 36.

[0062] As FIG. 1 shows, rearward movement of the sheath handle 28 (i.e.,away from the introducer 14) retracts the slidable sheath 16 away fromthe basket structure 36. This removes the compression force, and thebasket structure 36 opens to assume its prescribed three dimensionalshape.

[0063] The probe 18 also preferably includes a sliding hemostat sheath40. The physician slides the sheath 40 about the basket structure 36 toprotect it during its advancement through the introducer 14. Once thebasket structure 36 enters the guide sheath 16, the physician slides thehemostatic sheath 40 away rearward toward the probe handle 34. Furtherdetails of the use of the sheath 40 are disclosed in theabove-identified pending Patent Application.

[0064] The basket structure 36 can itself be variously constructed. Inthe illustrated and preferred embodiment (see FIG. 3), the basketstructure 36 comprises a base member 42 and an end cap 44. Generallyflexible splines 46 extend in a circumferentially spaced relationshipbetween the base member 42 and the end cap 44.

[0065] In the illustrated embodiment, eight, rectilinear splines 46 formthe basket structure 36. However, additional or fewer splines 46 couldbe used, as could splines of different configurations.

[0066] In this arrangement, the splines 46 are preferably made of aresilient inert material, like Nitinol metal or silicone rubber. Thesplines 46 are connected between the base member 42 and the end cap 44in a resilient, pretensed condition, shown in FIG. 3.

[0067] As FIG. 1 shows, the resilient splines 46 bend and conform to theendocardial tissue surface they contact. As FIG. 2 shows, the splines 46also collapse into a closed, compact bundle in response to the externalcompression force of the sliding sheath 18.

[0068] In the illustrated embodiment (see FIG. 4), each spline 46carries eight electrodes 38. Of course, additional or fewer electrodes38 can be used.

[0069] As will be described later, the system 10 can be operated ineither a unipolar mode or a bipolar mode. The basket electrodes 38 cantherefore be arranged in thirty-two bi-polar pairs, or as sixty-fouruni-polar elements.

[0070] In the illustrated and preferred embodiment (as FIG. 4 bestshows), the electrodes 38 are mounted to each spline 46 to maximizesurface contact to endocardial tissue, while at the same time minimizingexposure to the surrounding blood pool. Incidental exposure of theelectrodes 38 to blood while in contact with heart tissue introduces anunwanted artifact to E-Characteristic measurement, because theresistivity of blood is about three times lower than the resistivity ofheart tissue. This artifact can skew the E-Characteristic measurement toa lower value, thereby reducing the desired contrast between healthy andinfarcted tissue.

[0071] In the preferred embodiment (see FIG. 4), the electrodes 38 aremade of platinum or gold plated stainless steel bands affixed to onlyone side of the splines 46. This is the side of the spline 46 that, inuse, contacts endocardial tissue. The opposite surface of the splines 46(which, in use, contacts the blood pool) is free of electrodes.

[0072] In an alternative arrangement (see FIGS. 5 and 6), the electrodes38 can take the form of rings that encircle the entire spline 46. Inthis arrangement, the rear side of the electrodes 38, which during useface the blood pool, are coated with an electrically insulating material49 to prevent current transmission into blood.

[0073] It is believed that no more than 20% of the electrode surfaceshould be exposed to the blood pool during use. Preferable, less than 5%of the electrode should be so exposed during use.

[0074] In an alternative arrangement (see FIGS. 6A to 6C), one or moreof electrodes 38 can be introduced into the heart chamber through a veinor artery on a single flexible electrode support body 300, and not on abasket structure like that earlier described. The body 300 isillustrative of a family of flexible, elongated electrode supports ofvarious alternative constructions. In the preferred and illustratedembodiment, the body 300 is about 1 to 2.5 mm in diameter and about 1 to5 cm long.

[0075] As FIG. 27 shows, the body 300 is carried at the distal end of acatheter tube 302 used to guide the body 300 into the heart. A handle304 is attached to the proximal end of the catheter tube 302. The handle304 and catheter tube 302 carry a steering mechanism 306 for selectivelybending or flexing the support body 300 along its length, as the arrowsin FIG. 6A show.

[0076] The steering mechanism 306 can vary. In the illustratedembodiment (see FIG. 6C), the steering mechanism 306 includes a rotatingcam wheel 308 with an external steering lever 310 (as FIG. 6A shows). AsFIG. 6C shows, the cam wheel 308 holds the proximal ends of right andleft steering wires 312. The wires 312 pass through the catheter tube302 and connect to the left and right sides of a resilient bendable wireor spring (not shown) within the ablating element support body 300.

[0077] As FIG. 6A shows, movement of the steering lever 310 flexes orcurves the support body 300 from a generally straight configuration(shown in phantom lines in FIGS. 6A and 6B) into a generally arcuatecurve (shown in solid lines in FIGS. 6A and 6B). Through flexing, theelectrodes 38 can also be brought into conforming, intimate contactagainst the endocardial tissue, despite the particular contours andgeometry that the wall presents.

[0078] As shown in FIG. 6B, the electrodes 38 comprise rings encirclingthe support body 300. In this arrangement, the rear sides of theelectrodes 38, which, in use, face the blood pool, are preferably coatedwith the electrical insulating material 49 for the reasons stated above.Alternatively, the electrodes 38 can be affixed only to thetissue-contacting side of the support body 300, thereby making the rearside of the support body 300 free of electrodes 38, like the rectilinearspline 46 shown in FIG. 4.

[0079] The electrodes 38 carried by the support body 300, as FIG. 6Bshows, can by used in association with the process controller 20 to takeone or more E-Characteristic measurements, just as the electrodescarried by the basket structure. The support body 300 can be movedsequentially to different endocardial sites to obtain a plurality ofE-Characteristic measurements, which can be processed in the same manneras those taken by the stationary basket structure.

[0080] Further details of flexible electrode carrying elements can befound in copending U.S. patent application Ser. No. 08/138,142, filedOct. 15, 1993, entitled “Systems and Methods for Creating Long, ThinLesions in Body Tissue.”

[0081] In the illustrated embodiments (see FIGS. 4 and 6), a signal wire47 made from a highly conductive metal, like copper, leads from eachelectrode 46 (these signal wires are also shown diagrammatically in FIG.11). The signal wires 47 extend down the associated spline 46, by thebase member 42, and into the catheter body 32. An inert plastic wrapping43 preferably covers each spline 46 and electrode support body 300,except where the electrodes 38 project, to shield the signal wires.

[0082] The eight signal wires 47 for each spline 46 are twisted togetherto form a common bundle. The eight common bundles (not shown) are, inturn, passed through the catheter body 32 of the mapping probe 18. Thecommon bundles enter the probe handle 34.

[0083] The sixty-four signal wires 47 are connected within the probehandle 34 to one or more external connectors 48, as FIG. 3 shows. In theillustrated embodiment, each connector contains thirty-two pins toservice thirty-two signal wires.

[0084] In an alternative arrangement (not shown), the electrodes 38 canbe connected to a multiplexer/demultiplexer (M/DMUX) block (not shown)to reduce the number of signal wires carried by the catheter body 32.The M/DMUX block can comprise a multi-die integrated circuit mounted ona flexible support and wrapped about the catheter body 32. Thesignal-to-noise-ratio is thereby improved.

III. Measuring and Mapping the Tissue E-Characteristic

[0085] The system 10 transmits electrical current in a selected mannerthrough the basket electrodes 38 in contact with endocardial tissue.From this, the system 10 acquires impedance information about the hearttissue region that the basket electrodes 38 contact. The system 10processes the impedance information to derive the E-Characteristic,which assists the physician in identifying regions of infarcted tissuewhere ablation therapy may be appropriate.

[0086] For these purposes (see FIG. 3), the system 10 includes theprocess controller 20. The process controller 20 includes a currentgenerator module 50 and a signal processor module 52. The connectors 48electrically couple the basket electrodes 38 to both the generatormodule 50 and the processor module 52.

A. The Current Generator Module

[0087] The generator module 50 conveys a prescribed current signal toindividual basket electrodes 38.

[0088] In the illustrated and preferred embodiment (see FIG. 7), thegenerator module 50 includes an oscillator 54 that generates asinusoidal voltage signal. An associated interface 56 has a bus 58 thatcontrols the frequency of the output voltage signal and a bus 60 thatcontrols the amplitude of the output voltage signal. The interface 56,in turn, is programmed by a host processor 206, which will be describedin greater detail later.

[0089] The oscillator 54 has as an output stage that includes avoltage-to-current converter 62. In conventional fashion, the converter62 converts the sinusoidal voltage signal to current.

[0090] In the illustrated and preferred embodiment, the transmittedcurrent has an amplitude of about 0.1 milliamps to 5.0 milliamps. Thelower range of the current amplitude is selected to be high enough toovercome the influence of the double layer at the tissue-electrodeinterface on the E-Characteristic measurement. The high range of thecurrent amplitude is selected to avoid the induction of fibrillation.

[0091] The current has a frequency in a range of about 5 to 50 kHz. Therange is selected to avoid the induction of fibrillation, as well asprovide contrast between infarcted tissue and healthy tissue. The outputof the converter 62 can comprise a constant current with a constantfrequency within the above range. Alternatively, the interface 56 cancontrol the modulation of the frequency of the current signal within theprescribed range. Deriving tissue E-Characteristic by transmittingcurrents with different frequencies better differentiates amongdifferent tissue morphologies. It has been determined that lowerfrequencies within the range provide E-Characteristics yielding greaterquantitative contrast between infarcted and healthy tissues than higherfrequencies in this range.

[0092] The current output of the module 50 is supplied to the basketelectrodes 38 via supply path 68 through a switching element 64. Theinterface 56 electronically configures the switching element 64 todirect current in succession to selected basket electrodes 38 throughtheir associated signal wires in either a unipolar mode or a bipolarmode. Line 66 constitutes the control bus for the switching element 64.

[0093] As FIG. 8 shows, when operated in a unipolar mode, the currentreturn path 70 to the generator module 50 is provided by an exteriorindifferent electrode 72 attached to the patient.

[0094] When operated in a bipolar mode, the current return path 70 isprovided by an electrode carried on the basket structure 36 itself. Inthe illustrated and preferred embodiment, the bipolar return electrodeis either located immediately next to or three electrodes away from theselected transmitting basket electrode along the same spline. The firstcircumstance (shown in FIG. 9) will be called the Bipolar Two ElectrodeMode. The second circumstance (shown in FIG. 10) will be called theBipolar Four Electrode Mode.

[0095] The configuration of the switching element 64 can vary. FIG. 11diagrammatically shows one preferred arrangement.

[0096]FIG. 11 shows for illustration purposes a spline 46 with sevenadjacent electrodes 38, designated E1 to E7. Each electrode E1 to E7 iselectrically coupled to its own signal wire, designated W1 to W7. Theindifferent electrode, designated EI in FIG. 11, is also electricallycoupled to its own signal wire WI.

[0097] In this arrangement, the switching element 64 includes anelectronic switch S_(M) and electronic switches S_(E1) to S_(E7) thatelectrically couple the current generator to the signal wires W1 to W7.The switch S_(M) governs the overall operating mode of the electrodes E1to E7 (i.e., unipolar or bipolar). The switches S_(E1) to S_(E7) governthe electrical conduction pattern of the electrodes E1 to E7.

[0098] The switches S_(M) and S_(E1 to E7) are electrically coupled tothe current source. The supply path 68 of the generator module 50 iselectrically coupled to the leads L1 of the switches S_(E1 to E7) Thereturn path 70 of the generator module 50 is electrically coupled to thecenter lead L2 of the mode selection switch S_(M). A connector 67electrically couples the leads L3 of the switches S_(M) andS_(E1 to E7).

[0099] The center leads L2 of the selecting switches S_(E1 to E7) aredirectly electrically coupled to the signal wires W1 to W7 serving theelectrodes E1 to E7, so that one switch S_(E(N)) serves only oneelectrode E_((N)).

[0100] The lead L1 of the switch S_(M) is directly electrically coupledto the signal wire WI serving the indifferent electrode EI.

[0101] The interface 56 electronically sets the switches S_(M) andS_(E1 to E7) among three positions, designated A, B, and C in FIG. 12.

[0102] As FIG. 12 shows, Position A electrically couples leads L1 and L2of the associated switch. Position C electrically couples leads L2 andL3 of the associated switch. Position B electrically isolates both leadsL1 and L3 from lead L2 of the associated switch.

[0103] Position B is an electrically OFF position. Positions A and B areelectrically ON positions.

[0104] By setting switch S_(M) in Position B, the interface 56electronically inactivates the switching network 54.

[0105] By setting switch S_(M) in Position A, the interface 56electronically configures the switching element for operation in theunipolar mode. The center lead L2 of switch S_(M) is coupled to lead L1,electronically coupling the indifferent electrode EI to the return ofthe current generator. This configures the indifferent electrode EI as areturn path for current.

[0106] With switch S_(M) set in Position A, the interface 56electronically selectively configures each individual electrode E1 to E7to emit current by sequentially setting the associated switchS_(E1 to E7) in Position A. When the selected electrode E1 to E7 is soconfigured, it is electronically coupled to the supply of the currentgenerator and emits current. The indifferent electrode EI receives thecurrent sequentially emitted by the selected electrode E1 to E7.

[0107] By setting switch S_(M) in Position C, the interface 56electronically isolates the indifferent electrode EI from the electrodesE1 to E7. This configures the switching element for operation in thebipolar mode.

[0108] With switch S_(M) set in Position C, the interface 56 canelectronically alter the polarity of adjacent electrodes E1 to E7,choosing among current source, current sink, or neither.

[0109] By setting the selected switch S_(E1 to E7) in Position A, theinterface 56 electronically configures the associated electrode E1 to E7to be a current source. By setting the selected switch S_(E1 to E7) inPosition C, the interface 56 electronically configures the associatedelectrode E1 to E7 to be a current sink. By setting the selected switchS_(E1 to E7) in Position B, the interface 56 electronically turns offthe associated electrode E1 to E7.

[0110] In the Bipolar Two Electrode Mode, the interface 56 firstconfigures the electrode E1 to be a current source, while configuringthe immediate adjacent electrode E2 to be a current sink, while turningoff the remaining electrodes E3 to E7. After a preselected time period,the interface 56 then turns off electrode E1, configures electrode E2 tobe a current source, configures the next immediate adjacent electrode E3to be a current sink, while keeping the remaining electrodes E4 to E7turned off. After a preselected time period, the interface 56 then turnsoff electrode E2, configures electrode E3 to be a current source,configures the next immediate adjacent electrode E4 to be a currentsink, while keeping the remaining electrodes E1 and E5 to E7 turned off.The interface 56 cycles in this timed sequence until electrodes E6 andE7 become the current source/sink bipolar pairs (the remainingelectrodes E1 to E5 being turned off). The cycle can then be repeated,if desired, or ended after one iteration.

[0111] In the Bipolar Four Electrode Mode, the interface 56 firstconfigures the electrode E1 to be a current source, while configuringthe third adjacent electrode E4 to be a current sink, while turning offthe remaining electrodes E2, E3, and E5 to E7. After a predeterminedtime period, the interface 56 turns off electrode E1, configureselectrode E2 to be a current source, configures the next third adjacentelectrode E5 to be a current sink, while keeping the remainingelectrodes E3, E4, E6, and E7 turned off. After a predetermined timeperiod, the interface 56 turns off electrode E2, configures electrode E3to be a current source, configures the next third adjacent electrode E6to be a current sink, while keeping the remaining electrodes E1, E2, E4,E5, and E7 turned off. The interface 56 cycles in this timed sequenceuntil electrodes E4 and E7 become the current source/sink bipolar pairs(the remaining electrodes E1 to E3, E5, and E6 being turned off. Thecycle can then be repeated, if desired, or ended after one iteration.

[0112] In the preferred embodiment, there is a switching element 64 forthe electrodes on each basket spline, with the interface 56independently controlling each switching element.

B. Computing Tissue E-Characteristic

[0113] As FIG. 13 shows, the signal processor module 52 includes a dataacquisition system 74. While current is emitting by a selected basketelectrode, the system 74 senses the voltage in the tissue path usingselected electrodes on the basket 36.

[0114] Based upon the data acquired by the system 74, the host processor206 computes the E-Characteristic of the tissue path as follows:

[0115] (1) When operated in the Unipolar Mode, the E-Characteristic isthe impedance of the tissue path, computed based upon the followingequation:${{Impedance}({ohms})} = \frac{{PathVoltage}\quad ({volts})}{{PathCurrent}\quad ({amps})}$

[0116] The PathVoltage and PathCurrent are both root mean squared (RMS)values.

[0117] In the unipolar mode (see FIG. 8), the voltage is measuredbetween each transmitting electrode and the indifferent electrode (orbetween EI and E(n), where n represents the location of the currentemitting electrode). The impedance computed by the host processor 206 inthis mode reflects not only the impedance of the underlying myocardialtissue, but also includes the impedance of the other tissue mass in thepath. The computed impedance in this mode therefore is not the actualimpedance of the myocardial tissue itself. Rather, it provides arelative scale of impedance (or E-Characteristic) differences of themyocardial tissue lying in contact with the spline electrodes.

[0118] (2) When operated in the Bipolar Mode, the E-Characteristic ofthe tissue is the resistivity of the tissue path, computed as follows:

Resistivity(ohm·cm)=Impedance(ohm)×k(cm)

[0119]${{Impedance}({ohms})} = \frac{{PathVoltage}\quad ({volts})}{{PathCurrent}\quad ({amps})}$

[0120] where k is a dimensional constant (in cm) whose value takes intoaccount the methodology employed (i.e. either Bipolar Two Electrode Modeor Bipolar Four Electrode Mode) and the geometry of the electrode array(i.e., the size and spacing of the electrodes).

[0121] In general, k is approximately equal to the average crosssectional area of the current path divided by the distance between thevoltage sensing electrodes. The accuracy of the k value can be furtherimproved, if desired, empirically or by modeling.

[0122] The PathVoltage and PathCurrent are both root mean squared (RMS)values.

[0123] When operated in the Bipolar Two Electrode Mode (see FIG. 9), thevoltage is measured between the two adjacent current emitting/receivingelectrodes (or between E(n) and E(n+1)). When operated in the BipolarFour Electrode Mode (see FIG. 10), the voltage is measured between thetwo adjacent electrodes lying in between the current transmittingelectrode and the third adjacent return path electrode (or betweenE(n+1) and E(n+2)).

[0124] In either Bipolar Mode, the resistivity computed by the processor206 reflects the actual resistivity of the myocardial tissue lying incontact with the spline electrodes. However, the Bipolar Two ElectrodeMode is more prone to electric artifacts than the Bipolar Four ElectrodeMode, such as those due to poor electrical contact between electrode andtissue.

[0125] As FIG. 14 shows, the voltage signals sensed by the basketelectrodes 38 are passed back through the switching element 64 to thedata acquisition system 74. As FIG. 11 shows, a signal conditioningelement 224 preferably corrects alterations to the signal-to-noise ratiooccurring in the voltage signals during propagation through the probebody 32.

[0126] The data acquisition system 74 includes a multiplexer 76 thatselects and samples in succession the voltage associated with eachtransmitting electrode E(n) carried by the basket structure 36. For eachselected current transmitting electrode E(n), the multiplexer 76 samplesfor a prescribed time period the analog sinusoidal voltage measuredbetween the sensing electrodes.

[0127] A sample and hold element 80 stores the sampled analog voltagesignals. The stored signals are sent to an analog-to-digital (A-to-D)converter 82, which converts the sampled voltage signals to digitalsignals. The multiplexer 76 makes possible the use of a singleanalog-to-digital conversion path.

[0128] The digital signals are sent to a host processor 206 through aninterface 226. The host processor 206, based upon a conventional sortingscheme, obtains the peak voltage and, from that, computes the RMSvoltage. The host processor 206 then computes the E-Characteristic,using the RMS voltage and RMS current (and, for the Bipolar Mode, theconstant k) as described above. The RMS current is known by theprocessor 206, since it has been programmed by it through the interface56 (see FIG. 7).

C. Processing the E-Characteristic

[0129] The computed E-Characteristic values can be processed by thesystem 10 in various ways.

[0130] In one embodiment (see FIG. 13), the signal processor moduleincludes means 90 for sorting the multiple computed E-Characteristicvalues in absolute terms, arranging them according to a preassignedelectrode numbering sequence, representing relative electrode position.

[0131] The means 90 can create as an output a table (either as a graphicdisplay or in printed form), as follows: TABLE 1 SPLINE ELECTRODE E-CHARS1 El 75 S1 E2 114 S1 E3 68 S1 E4 81 S2 E1 69 S2 E2 71 S2 E3 67 S2 E4 66S3 E1 123 S3 E2 147 S3 E3 148 S3 E4 140 ... etc ... ... etc ... ... etc...

[0132] In Table 1, the spline elements of the basket are identified asS1, S2, S3, etc. The electrodes carried by each spline element arenumbered from the distal end as E1, E2, E3, and so on. TheE-Characteristic values are expressed in terms of resistivity (ohm·cm).The values expressed are idealized and given for illustration purposes.In addition, or alternatively, the means 90 can also create as an outputa two or three dimensional display that spatially maps the relativeposition of the computed absolute resistivity values, based upon basketelectrode positions.

[0133]FIG. 15 shows a representative display of E-Characteristics(expressed as resistivity values), based upon the data listed inTable 1. In FIG. 15, circled Area A identifies a region of low relativetissue resistivity, indicative of infarcted heart tissue. Area B in FIG.15 is a region of normal tissue resistivity, indicative of healthy hearttissue.

[0134] Preferably, the signal processor module 52 also includes means 92(see FIG. 13) for arranging the derived absolute E-Characteristics intogroups of equal E-Characteristic values for display in spatial relationto the location of the electrodes 38. This output better aids thephysician in interpreting the E-Characteristics, to identify the regionsof low relative tissue E-Characteristics, where ablation may beappropriate.

[0135] As FIG. 16 shows, the means 92 includes a processing step thatcomputes the location of the electrodes 38 in a three dimensionalcoordinate system. In the illustrated and preferred embodiment, a threedimensional spherical coordinate system is used.

[0136] The means 92 next includes a processing step that generates bycomputer a three dimensional mesh upon the basket surface. The pointswhere the mesh intersect are called nodes. Some of the nodes willoverlie the electrodes on the basket. These represent knots, for whichthe values of the E-Characteristic are known.

[0137] The values of the E-Characteristic for the remaining nodes of thethree dimensional mesh have not been directly measured. Still, thesevalues can be interpolated at each remaining node based upon the knownvalues at each knot.

[0138] One method of doing this interpolation is using three dimensionalcubic spline interpolation, although other methods can be used. Thecubic spline interpolation process is incorporated in the MATLAB™program, sold by The MathWorks Incorporated.

[0139] The means 92 creates an output display by assigning onedistinguishing idicium to the maximum E-Characteristic value (whetheractually measured or interpolated) and another distinguishing idicium tothe minimum E-Characteristic value (again, whether actually measured orinterpolated). In the illustrated and preferred embodiment, thedistinguishing indicia are contrasting colors or shades.

[0140] The means 92 assigns computer generated intermediate indicia tointermediate measured and interpolated values, based upon a linearscale. In the illustrated and preferred embodiment, the intermediateindicia are color hues between the two contrasting colors or shades.

[0141] The means 92 projects the generated color (or selected indicia)map upon the basket surface, based upon location of the nodes in thethree dimensional mesh. The means 92 thus creates as an output a displayshowing iso-E-Characteristic regions.

[0142]FIG. 17 shows a representative display of iso-resistivity regions,based upon the idealized, illustrative data listed in Table 1.

D. Matching E-Characteristic and Tissue Conductivity

[0143]FIG. 18 shows another embodiment of a process controller 200 thatcan be used in association with the probe 18, as already described.

[0144] The process controller 200 in FIG. 18, like the processcontroller 20 shown in FIG. 3, includes the current generator module 50and the signal processing module 52 for deriving and processing tissueE-Characteristics in the manners previously discussed.

[0145] In addition, the process controller 200 in FIG. 18 includes amodule 202 for pacing the heart to acquire electrograms in aconventional fashion. The pacing module 202 is electrically coupled tothe probe connectors 48 to provide a pacing signal to tone electrode 38,generating depolarization foci at selected sites within the heart. Thebasket electrodes 38 also serve to sense the resulting electrical eventsfor the creation of electrograms.

[0146] Operation of the pacing module 202 is not required whenventricular tachycardia (VT) is either purposely induced (e.g., byprogrammed pacing) or occurs spontaneously. In this situation, thedeployed basket electrodes 38 sense the electrical events associatedwith VT itself.

[0147] The process controller 200 in FIG. 18 further includes a secondsignal processing module 204 for processing the electrogram morphologiesobtained from the basket electrodes 38.

[0148] The process controller 200 in FIG. 18 also includes a hostprocessor 206 that receives input from the data acquisition system 74and the electrogram processing module 204. The processor 206 analyzesthe tissue E-Characteristic and electrogram information to compute amatched filtered output, which further enhances the CIR of ablation siteidentification.

[0149] The modules 202, 204, and 206 may be configured in various ways.

[0150] In the illustrated and preferred embodiment (see FIG. 19), thepacing module 202 includes a controller interface 208 coupled to thehost processor 206, which will be described in greater detail later. Thecontroller interface 208 is also coupled to pulse generator 210 and anoutput stage 212.

[0151] The output stage 212 is electrically coupled by supply path 220and return path 218 to the same switching element 64 as the currentgenerator module 50. The switching element 64 has been previouslydescribed and is shown schematically in FIG. 11. As FIG. 11 shows inphantom lines, the pacing module 202 and current generator module 50 areconnected to the switching element 64.

[0152] The controller interface 208 includes control buses 214, 216, and218. Bus 214 conveys pulse period control signals to the pulse generator210. Bus 216 conveys pulse amplitude control signals to the pulsegenerator 210. Bus 219 constitutes the control bus path for theswitching element 64.

[0153] When used to pace the heart, the switching element 64 distributesthe signals generated by the pacing module 202 to selected basketelectrodes 38. The pacing sequence is governed by the interface 208,which the host processor 206 controls.

[0154] The resulting electrogram signals sensed by the basket electrodes38 are also passed back through the switching element 64 to the hostprocessor 206 and the processing module 204 through the same analogprocessing path as the E-Characteristic signals, as FIG. 11 shows, andas already described.

[0155]FIG. 20 schematically shows the components of the host processor206 and the electrogram processing module 204.

[0156] The host central processing unit (CPU) 206 communicates with amass storage device 230 and an extended static RAM block 232. A userinteractive interface 234 also communicates with the CPU 206.

[0157] As FIG. 20 shows, the interactive user interface 234 includes aninput device 244 (for example, a key board or mouse) and an outputdisplay device 246 (for example, a graphics display monitor or CRT).

[0158] The CPU 206 also communicates with the current generator module50; pacing module 202 and the interface 226 for the system 74, aspreviously described. In this way, the CPU 206 coordinates overallcontrol functions for the system 10.

[0159] As FIG. 20 shows, the electrogram processing module 204 includesa bus 235 and a bus arbiter 236 that receive the digital output of theA-to-D converter 82 through the interface 226. The bus arbiter 236arbitrates the distribution of the digital electrogram morphologysignals to one or more digital signal processors 238, which also form apart of the processing module 204 and which also communicate with theCPU 206.

[0160] The illustrated and preferred embodiment employs four signalprocessors 238 operating concurrently, but different numbers ofprocessors 238 can be used. If N is the total number of basketelectrodes and M is the number of processors 238, then each processor238 is responsible for processing the signals coming from N/M electrodesin the Unipolar Mode and N/(2M) electrodes in the Bipolar Two or FourMode.

[0161] To speed up data processing, each processor 238 includes a staticRAM block 240. The data is processed real-time and stored in the blocks240.

[0162] The signal processors 238 include various means for processingthe electrogram signals as follows:

[0163] (i) to detect the earliest depolarization event;

[0164] (ii) to construct from the electrogram signals iso-chronal oriso-delay maps of the depolarization wavefronts, depending upon how theelectrograms are obtained, which can be presented on the display device246 for viewing by the physician; and

[0165] (iii) to construct from the electrogram signals iso-conductionmaps, which can also be presented on the display device 246 for viewingby the physician.

[0166] The CPU 206 employs additional means for processing theelectrogram signals and the E-Characteristic signals as follows:

[0167] (iv) to match the iso-conduction maps with theiso-E-Characteristic maps, which can be presented on the display device246 for viewing by the physician; and

[0168] (v) based upon the matched output of (iv), to identify apotential ablation site.

(i) Identifying the Earliest Depolarization Event

[0169]FIG. 21B shows the means 250 for detecting the earlydepolarization event.

[0170] The CPU 206 displays the electrograms on the display 246 of theinteractive user interface 234 (see FIG. 21A). After analyzing thedisplay 246, the physician can manually choose a reference time forconventional electrogram beat clustering purposes. The physician can usethe mouse or a keyboard device 244 for this purpose.

[0171] In the situation where ventricular tachycardia is purposelyinduced or is occurring spontaneously, the electrogram beats areclustered relative to the reference time to compute the propagation timewhen an electrogram for ventricular tachycardia is sensed by eachelectrode 38. For all the beats in the selected cluster, the physicianmanually selects the earliest depolarization event for each electrode38. The interactive interface 234 transmits the physician's choice tothe host CPU 206, which creates a matrix of the computed propagationtimes.

[0172] In the situation where the heart is being paced by the module202, the beats are clustered relative to the reference time forcomputing the activation delay for each electrogram. The activationdelay is measured between the pacing pulse and the earliestdepolarization event. For all the beats in the selected cluster, thephysician manually selects the earliest depolarization event for eachelectrode 38. In this situation as before, the interactive interface 234transmits the physician's choice to the host CPU 206, which creates amatrix of the computed activation delays.

[0173]FIG. 21A shows four representative electrograms of a heartundergoing VT. FIG. 21A shows the reference time selected for beatclustering purposes and the early depolarization events selected for thepurpose of illustration. From this, the propagation times t₁; t₂; t₃; t₄can be computed as the differences between the time of thedepolarization event and the reference time in each electrogram.

(ii) Constructing an Iso-Chronal or Iso-Delay Displays

[0174]FIG. 22 shows the means 252 for creating either an iso-chronaldisplay of the propagation times (when VT is induced or spontaneouslyoccurs) or an iso-delay display of activation times (when the module 202is used to pace the heart). For purposes of description, each will becalled the “computed electrogram event.”

[0175] The means 252 generally follows the same processing steps as themeans 92 (see FIG. 16) for creating the iso-E-Characteristic display.

[0176] The means 252 includes a processing step that computes thelocation of the electrodes in a spherical coordinate system.

[0177] The means 252 next generates by computer a three dimensional meshupon the basket surface. The points where the mesh intersect are callednodes. Some of the nodes overlie the electrodes on the basket. Theserepresent knots, for which the values of the computed electrogram eventare known.

[0178] The values of the computed electrogram event for the remainingnodes of the three dimensional mesh have not been directly measured.Still, these values can be interpolated at each remaining node basedupon the known values at each knot.

[0179] As before, three dimensional cubic spline interpolation can beused, although other methods can be used.

[0180] The means 252 creates an output display on the device 246 byassigning one color the maximum value of the computed electrogram event(whether actually measured or interpolated) and another color to theminimum value of computed electrogram event (again, whether actuallymeasured or interpolated). Computer generated intermediate hues betweenthe two colors are assigned by the host CPU 206 to intermediate measuredand interpolated values, based upon a linear scale.

[0181] The means 252 projects the generated color map upon the basketsurface, based upon location of the nodes in the three dimensional mesh.

[0182]FIG. 23 shows a representative display generated according to thisprocessing means. The CPU 206 generates this display on the displaydevice 246 for viewing by the physician.

[0183] A potential ablation site can be identified at regions where arapid transition of hues occurs. Area A on FIG. 23 shows such a region.

[0184] When the electrograms used for beat clustering show an induced orspontaneous VT, the resulting display is an iso-chronal map of theexamined tissue region. When the electrograms used for beat clusteringare based upon a paced heart, the display is an iso-delay map of theexamined tissue region.

(iii) Creating Iso-Conduction Display

[0185]FIG. 24 shows the means 254 for creating an iso-conductiondisplays of the computed electrogram event.

[0186] An iso-conduction display more rapidly identifies the regions ofslow conduction which are candidate ablation sites, than an iso-chronalor iso-delay display. The iso-conduction display requires lesssubjective interpretation by the physician, as the regions of slowconduction stand out in much greater contrast than on an iso-chronal oriso-delay display.

[0187] The means 254 draws upon the same input and follows much of thesame processing steps as the means 252 just described. The means 254computes the location of the electrodes in a spherical coordinate systemand then generates a three dimensional mesh upon the basket surface. Themeans 254 interpolates the computed electrogram event for the nodesbased upon the known values at the knots.

[0188] Unlike the previously described means 252, the means 254 computesthe inverse of the magnitude of the spatial gradient of the computedelectrogram event. This inverse spatial gradient represents the value ofthe conduction of the cardiac signal in the examined tissue.

[0189] To carry out this processing step, the means 254 first computesthe spatial gradient computed electrogram event for each node of themesh. The methodology for making this computation is well known.

[0190] Next, the means 254 computes the magnitude of the spatialgradient, using, for example, known three dimensional vector analysis.Then, the means 254 computes the inverse of the magnitude, whichrepresents the conduction value.

[0191] The means 254 clips all magnitudes larger than a predeterminedthreshold value, making them equal to the threshold value. Thisprocessing step reduces the effects of inaccuracies that may ariseduring the mathematical approximation process.

[0192] The computation of conduction (i.e., the velocity of thepropagation) can be exemplified for the case when propagation times areprocessed. By substituting the activation delays for propagation times,one can compute the conductions for data obtained from paced hearts.

[0193] The location of any point on the three-dimensional mesh shown inFIG. 25 is given by the azimuth angle, φ and the elevation angle, δ. Theradius of the underlying surface is normalized to one. The conduction isdefined by EQUATION (1): $\begin{matrix}{{{Conduction}\quad \left( {\phi,\delta} \right)} = {\frac{dspace}{d\quad {Prop\_ Time}\quad \left( {\phi,\delta} \right)}}} & \text{EQUATION (1)}\end{matrix}$

[0194] Given that the radius of the meshed surface is one, one obtainsthe spatial gradient of propagation times: $\begin{matrix}{\frac{d\quad {Prop\_ Time}\quad \left( {\phi,\delta} \right)}{dspace} = {{\frac{{\partial\quad {Prop\_ Time}}\quad \left( {\phi,\delta} \right)}{\partial\phi} \times \Phi} + {\frac{{\partial\quad {Prop\_ Time}}\quad \left( {\phi,\delta} \right)}{\partial\delta} \times \Delta}}} & \text{EQUATION (2)}\end{matrix}$

[0195] where Φ and Δ are unity vectors of the spherical coordinatesystem defining the directions of the azimuth and elevation,respectively.

[0196] Thus, the conduction can be computed using EQUATION (3):$\begin{matrix}{{{Conduction}\quad \left( {\phi,\delta} \right)} = \frac{1}{\sqrt{\left( \frac{{\partial\quad {Prop\_ Time}}\quad \left( {\phi,\delta} \right)}{\partial\phi} \right)^{2} + \left( \frac{{\partial\quad {Prop\_ Time}}\quad \left( {\phi,\delta} \right)}{\partial\delta} \right)^{2}}}} & \text{EQUATION (3)}\end{matrix}$

[0197] which is actually the inverse of the spatial gradient magnitude.When the conduction is numerically approximated, the derivatives inEQUATION (3) can be computed by any numerical method appropriate for theestimation of first derivatives.

[0198] The means 254 creates a display by assigning one color thethreshold conduction value (i.e., the maximum permitted value) andanother color to the minimum conduction value. Computer generated huesare assigned to intermediate values, based upon a linear scale, as abovedescribed.

[0199] The means 254 projects the generated color map upon the basketsurface, based upon location of the nodes in the three dimensional mesh.

[0200]FIG. 25 shows a representative iso-conduction display generatedaccording to the just described methodology and using the same data asthe iso-chronal display shown in FIG. 23. The CPU 206 generates thisdisplay on the display device 246 for viewing by the physician.

[0201] Area A in FIG. 25 shows a region of slow conduction, whichappears generally at the same location as the rapid hue transition inFIG. 23 (also identified as Area A). FIG. 25 shows the more pronouncedcontrast of the region that the iso-conduction display provides, whencompared to the iso-chronal display of FIG. 23. Thus, the iso-conductiondisplay leads to a more certain identification of a potential ablationsite.

(iv) Matching Iso-Conduction with Iso-E-Characteristic

[0202]FIG. 26 shows the means 256 for matching the iso-conduction withthe iso-E-Characteristic for the analyzed heart tissue.

[0203] The means 256 derives the values of the E-Characteristic at thenodes of three dimensional mesh in the same manner already described.Next, the means 256 normalizes these E-Characteristic values into anarray of numbers from 0.0 to 1.0. The number 1.0 is assigned to theabsolute lowest E-Characteristic value, and the number 0.0 is assignedto the absolute highest E-Characteristic value. E-Characteristic valuesbetween the absolute lowest and highest values are assigned numbers on alinear scale between the lowest and highest values.

[0204] The means 256 also derives the values of the computed electrogramevent at the nodes of three dimensional mesh in the manner alreadydescribed. The means 256 computes the inverse of the magnitude ofspatial gradient of the computed electrogram event, as previouslydescribed, to derive the value of the conduction of the cardiac signalin the examined tissue.

[0205] The means 256 then normalizes these conduction values into anarray of numbers from 0.0 to 1.0. The number 1.0 is assigned to theabsolute lowest conduction value, and the number 0.0 is assigned to thethreshold conduction value. As before, conduction values between theabsolute lowest and highest values are assigned numbers on a linearscale between the lowest and highest values.

[0206] The means 256 then applies, using known mathematicalcomputational techniques, a two dimensional matched filtering process tothe normalized conduction data using the normalized E-Characteristicdata as a template, or vice versa. Alternatively, a two dimensionalcross-correlation can be applied to the normalized E-Characteristic andconduction. As used in this Specification, “matching” encompasses bothtwo dimensional matched filtering, two dimensional cross-correlation,and a like digital signal processing techniques.

[0207] The values obtained from the matched filtering process arenormalized, by dividing each value by the maximum absolute value. Afternormalization, the value will range between 0.0 and 1.0.

[0208] The means 256 creates a display by assigning one color thehighest normalized matched filter value and another color to the lowestnormalized matched filter value. Computer generated hues are assigned tointermediate values, based upon a linear scale, as above described.

[0209] The means 256 projects the generated color map upon the basketsurface, based upon location of the nodes in the three dimensional mesh.

[0210]FIG. 27 shows a representative display processed according to theabove methodology. The CPU 206 generates this display on the displaydevice 246 for viewing by the physician.

[0211] The display matches the normalized iso-conduction values with thenormalized iso-E-Characteristic values, in effect matching electrogramswith tissue E-Characteristics. This matching provides more precisedifferentiation between regions of infarcted tissue and regions ofhealthy tissue.

[0212] This information can be further processed to identify a potentialablation site to maximize the CIR.

(v) Identifying a Potential Ablation Site

[0213]FIG. 28 shows a means 258 for identifying a potential ablationsite based upon the matched output of the normalized conduction valuesand the normalized E-Characteristic values, generated by the means 256.

[0214] The means 258 selects a threshold value. Tissue regions havingmatched output values above the threshold constitute potential ablationsites. Locating an optimal threshold value can be done by empiricalstudy or modeling. The threshold value for a given set of data will alsodepend upon the professional judgment of the physician.

[0215]FIG. 29 shows a representative display processed according to theabove methodology. In FIG. 29, a threshold of 0.8 has been used forillustration purposes. Values greater than the threshold of 0.8 havebeen set to 1.0, while values equal to or less than 0.8 have been set to0.0. The CPU 206 generates this display on the display device 246 forviewing by the physician.

[0216]FIG. 29 provides by sharp contrast between black and white (withno intermediate hues) the potential ablation site (Area A).

E. Ablating the Tissue

[0217] Regardless of the specific form of the output used, the physiciananalyses one or more of the outputs derived from the basket electrodes38 to locate likely efficacious sites for ablation.

[0218] The physician can now takes steps to ablate the myocardial tissueareas located by the basket electrodes 38. The physician can accomplishthis result by using an electrode to thermally destroy myocardialtissue, either by heating or cooling the tissue. Alternatively, thephysician can inject a chemical substance that destroys myocardialtissue. The physician can use other means for destroying myocardialtissue as well.

[0219] In the illustrated embodiment (see FIG. 30), an externalsteerable ablating probe 100 is deployed in association with the basketstructure 36.

[0220] Various features of the invention are set forth in the followingclaims.

We claim:
 1. A system for examining tissue within the heart comprisingat least three spaced apart electrodes, means for locating at least twoof the electrodes within the heart for contact with endocardial tissue,generator means operable in one mode for transmitting electrical currentin a first path through a region of heart tissue between a first pair ofthe electrodes, at least one of which is within the heart, the generatormeans being operable in another mode for transmitting electrical currentin a second path through heart tissue in the region between a secondpair of the electrodes, at least one of which is within the heart,without substantially altering position of first pair of electrodes, andprocessing means for deriving a tissue electrical characteristic based,at least in part, upon sensing the impedances of the tissue lying in thefirst and second paths.
 2. A system according to claim 1 wherein theprocessing means compares the derived electrical characteristic of thetissue lying in first path with the derived electrical characteristic ofthe tissue lying second path.
 3. A system according to claim 2 whereinthe processing means generates an output based upon the comparison ofderived electrical characteristics.
 4. A system according to claim 1wherein the processing means derives the electrical characteristic bymeasuring the voltages in the first and second paths and dividing themeasured voltages by the measured currents transmitted through the pathsto derive the tissue impedances.
 5. A system according to claim 4wherein the processing means compares the derived impedance of thetissue lying in the first path with the derived impedance of the tissuelying in the second path.
 6. A system according to claim 5 wherein theprocessing means generates an output based upon the comparison of thederived tissue impedances.
 7. A system according to claim 1 wherein theprocessing means derives the resistivities of the tissue lying in thefirst and second paths.
 8. A system according to claim 7 wherein theprocessing means compares the derived resistivity of the tissue lying infirst path with the derived resistivity of the tissue lying second path.9. A system according to claim 8 wherein the processing means generatesan output based upon the comparison of the derived tissue resistivities.10. A system according to claim 1 wherein the locating means establishessubstantially simultaneous, constant contact between at least two of theelectrodes and endocardial tissue.
 11. A system according to claim 1 andwherein the locating means establishes substantially simultaneous,constant contact between at least two of the electrodes and endocardialtissue, and wherein at least one of the remaining electrodes comprisesan electrode located outside the heart.
 12. A system according to claim1 wherein the locating means establishes substantially simultaneous,constant contact between all the electrodes and endocardial tissue. 13.A system according to claim 1 wherein the locating means includes acatheter tube having a distal end that carries at least two of theelectrodes.
 14. A system according to claim 13 wherein the generatormeans and processing means includes a multiplexer/demultiplexer elementat least a portion of which is carried by the catheter tube.
 15. Asystem according to claim 1 and further including means for emittingenergy to ablate myocardial tissue within the heart.
 16. A system forexamining tissue within the heart comprising a three dimensional arrayof spaced apart electrodes for contacting endocardial tissue in aselected position, means for transmitting electrical current from thespaced apart electrodes in multiple paths through a region of hearttissue without altering the position of the array, and processing meansfor deriving an electrical characteristic of tissue lying in themultiple paths based, at least in part, by sensing tissue impedances inthe multiple paths.
 17. A system according to claim 16 wherein theprocessing means compares the electrical characteristic derived fortissue lying in one of the multiple paths with the electricalcharacteristic derived for tissue lying an another one of the multiplepaths.
 18. A system according to claim 17 wherein the processing meansgenerates an output based upon the comparison of the derived electricalcharacteristics.
 19. A system according to claim 16 wherein theprocessing means derives the electrical characteristic for each of themultiple paths by measuring the voltages in each path and dividing themeasured voltage by the measured current transmitted through the path toderive the tissue impedance in the path.
 20. A system according to claim16 wherein the processing means compares the derived tissue impedancesof the multiple paths.
 21. A system according to claim 20 wherein theprocessing means generates an output based upon the comparison of thederived tissue impedances.
 22. A system according to claim 16 whereinthe processing means derives the resistivities of the tissue lying ineach of the multiple paths.
 23. A system according to claim 22 whereinthe processing means compares the derived tissue resistivities of themultiple paths.
 24. A system according to claim 23 wherein theprocessing means generates an output based upon the comparison of thederived tissue resistivities.
 25. A system according to claim 16 andfurther including a catheter tube having a distal end that carries thethree dimensional array.
 26. A system according to claim 25 wherein themeans for transmitting electric current and the processing meansincludes a multiplexer/demultiplexer element at least a portion of whichis carried by the catheter tube.
 27. A system for examining tissuewithin the heart comprising at least three spaced apart electrodes,means for locating at least two of the electrodes within the heart forcontact with endocardial tissue, generator means operable in one modefor transmitting electrical current in a first path through a region ofheart tissue between a first pair of the electrodes, at least one ofwhich is within the heart, the generator means being operable in anothermode for transmitting electrical current in a second path through hearttissue in the region between a second pair of the electrodes, at leastone of which is within the heart, without substantially alteringposition of first pair of electrodes, processing means for deriving atissue electrical characteristic based, at least in part, upon sensingthe impedances of the tissue lying in the first and second paths, andmeans for generating an output of the derived electrical characteristicin spatial relation to the location of the first and second paths.
 28. Asystem according to claim 27 wherein the processing means derives theelectrical characteristic by measuring the voltages in the first andsecond paths and dividing the measured voltages by the measured currentstransmitted through the paths to derive the tissue impedances, andwherein the generated output includes the derived tissue impedances inspatial relation to the location of the first and second paths.
 29. Asystem according to claim 27 wherein the processing means derives theresistivities of the tissue lying in the first and second paths, andwherein the generated output includes the derived tissue resistivitiesin spatial relation to the location of the first and second paths.
 30. Asystem according to claim 27 wherein the generated output comprises atabular listing.
 31. A system according to claim 27 wherein thegenerated output comprises a graphic display.
 32. A system according toclaim 27 wherein the locating means establishes substantiallysimultaneous, constant contact between at least two of the electrodesand endocardial tissue.
 33. A system according to claim 27 and whereinthe locating means establishes substantially simultaneous, constantcontact between at least two of the electrodes and endocardial tissue,and wherein at least one of the remaining electrodes comprises anelectrode located outside the heart.
 34. A system according to claim 27wherein the locating means establishes substantially simultaneous,constant contact between all the electrodes and endocardial tissue. 35.A system according to claim 27 wherein the locating means includes acatheter tube having a distal end that carries at least two of theelectrodes.
 36. A system according to claim 35 wherein the generatormeans and processing means includes a multiplexer/demultiplexer elementat least a portion of which is carried by the catheter tube.
 37. Asystem according to claim 27 wherein the locating means includes a threedimensional structure for supporting at least two of the electrodes. 38.A system according to claim 37 wherein the locating means includes acatheter tube having a distal end that carries the three dimensionalstructure.
 39. A system according to claim 38 wherein the generatormeans and processing means includes a multiplexer/demultiplexer elementat least a portion of which is carried by the catheter tube.
 40. Asystem according to claim 27 and further including means for emittingenergy to ablate myocardial tissue within the heart.
 41. A method forexamining tissue within the heart comprising the steps of transmittingelectrical current in a first path through a region of tissue between afirst pair of electrodes, at least one of which is located within theheart in contact with endocardial tissue, transmitting electricalcurrent in a second path through tissue in the region between a secondpair of the electrodes, at least one of which is located within theheart in contact with endocardial tissue, without substantially alteringthe position of the first pair of electrodes, and deriving theelectrical characteristics of tissue lying in the first and second pathsbased, at least in part, upon sensing the impedances in the first andsecond paths.
 42. A method according to claim 41 and further includingthe step of comparing the derived electrical characteristic of the firstpath with the derived electrical characteristic of the second path. 43.A method according to claim 42 and further including the step ofcreating an output based upon the comparison of derived electricalcharacteristics.
 44. A method according to claim 41 and furtherincluding the step of creating an output of the derived electricalcharacteristics in spatial relation to the location of the first andsecond paths.
 45. A method according to claim 41 wherein, in the step ofderiving the electrical characteristic, voltages are measured in thefirst and second paths and the measured voltage in each path is dividedthe measured current transmitted through the associated path to derivetissue impedances.
 46. A method according to claim 45 and furtherincluding the step of comparing the derived tissue impedances.
 47. Amethod according to claim 46 and further including the step of creatingan output based upon the comparison of the derived tissue impedances.48. A method according to claim 46 and further including the step ofcreating an output of the derived tissue characteristics in spatialrelation to the location of the first and second paths.
 49. A methodaccording to claim 41 wherein, in the step of deriving the electricalcharacteristic, the resistivities of the tissue lying in the first andsecond paths are derived.
 50. A method according to claim 49 and furtherincluding the step of comparing the derived tissue resistivities.
 51. Amethod according to claim 50 and further including the step of creatingan output based upon the comparison of the derived tissue resistivities.52. A method according to claim 50 and further including the step ofcreating an output of the derived tissue resistivities in spatialrelation to the location of the first and second paths.
 53. A method forexamining tissue within the heart comprising the steps of positioning aarray of spaced apart electrodes in contact with a region of endocardialtissue in a desired position, transmitting electrical current from thespaced apart electrodes in multiple paths through a region of hearttissue without altering the position of the array, and deriving theelectrical characteristics of tissue lying in the multiple paths based,at least in part, to sensing tissue impedances in the multiple paths.54. A method according to claim 53 and further including the step ofcomparing the derived electrical characteristics of the multiple pathsto each other.
 55. A method according to claim 54 and further includingthe step of creating an output based upon the derived electricalcharacteristics.
 56. A method according to claim 54 and furtherincluding the step of creating an output of the derived electricalcharacteristics in spatial relation to the multiple paths.